The trend of vast penetration of distributed energy resources (DERs, such as PV or wind farms) in low and medium voltage power networks calls for a substantial improvement in the control methods of these systems due to the two conflicting contributions of DERs. On one hand, more flexibility is added to the networks, which allows for a better and more reliable operation on local scales. In particular, local power balances in low-voltage grids become possible, creating the so-called microgrids in the distribution networks. On the other hand, the high volatility of DERs can cause unpredictable reductions in the quality-of-supply. In this context, the local resilience of the system against major external disturbances (e.g., faults and blackouts) can be substantially improved if the microgrid is capable of performing an islanding maneuver (i.e., the disconnection from the main grid subsequent to an intentional or non-intentional decision, e.g., [7]).
Usually, the real-time control of microgrids is performed using droop controllers that react to frequency and voltage, while non real-time control decisions are taken by suitably defined management systems [8]. In this context, the strategy for an islanding maneuver relies on the availability of a classic slack resource with mechanical rotating inertia. Hence, the slack resource is normally predefined and, in case the islanding takes place when there is a large power import from the external grid, a shedding scheme may be required to avoid system collapse. Moreover, the sub-second control is not addressed directly, as it is left to the local droop controllers. The main advantages of this control strategy is its simplicity of implementation, as it relies on the fitting of few parameters, and that it inherently ensures that all droop-controlled units contribute to the power imbalance caused by the islanding.
In contrast, the main disadvantages are: the ignorance of the state of the pre-selected slack, which may be very dynamic, especially for electrochemical storage devices and the use of locally-controlled shedding schemes that may trigger all non-critical loads at a given frequency threshold.
Recently, a different framework for the real-time control of active distribution networks, and in particular microgrids with little or null inertia, has been proposed in [1]. With the Commelec framework, electrical resources in the microgrid are under the control of one or several grid agents, which define explicit power setpoints in real-time (i.e., every ˜0.1 sec). Contrary to classic strategies, this mode of operation exposes the state of all resources to the local grid controller, enabling an efficient and stable operation without large rotating masses. The framework is designed to be robust (i.e., it avoids the problems inherently posed by software controllers) and scalable (i.e., it easily adapts to grids of any size and complexity). It uses a hierarchical system of software agents, each responsible for a single resource (loads, generators and storage devices) or an entire subsystem (including a grid and/or a number of resources). It is abstract in the sense that it applies to all electrical subsystems and specifies their capabilities, expected behavior, and a simplified view of their internal state using a common, device-independent protocol.
With the present invention, one aim is to add to the Commelec real-time control framework the ability to support unintentional islanding.